The Domain Name System (DNS) is a system that stores information associated with domain names in a distributed database on one or more networks. The stored information includes the Internet Protocol (IP) address associated with a domain name. The domain name space may be thought of as a tree of domain names. Each node or leaf in the tree is associated with resource records, which hold information associated with the domain name. The tree is divided into zones. A zone is a collection of connected nodes that are authoritatively served by an authoritative DNS server. A DNS server may host one or more zones. Resolving a domain name may entail recursing through several DNS servers to find the needed information.
A domain name usually includes multiple parts separated by dots. For example, encarta.msn.com has three parts: encarta, msn, and com. The rightmost part is the top-level domain. In the example, the top-level domain is com. Each part to the left of the top-level domain is a sub-domain. In the example, msn is a second-level domain. The left-most part of the domain name is the host name. The host name specifies the name of a specific machine with a specific IP address in a domain. In the example, encarta is the host name.
In order for a host name to be resolved globally across multiple domain and zone boundaries, a machine has to register in all the domains, which greatly increases administration complexity. A host name that is needed globally cannot be added or managed easily or efficiently.